Light emitting diode (LED) lighting systems are becoming more prevalent as replacements for existing lighting systems. LEDs are an example of solid state lighting (SSL) and have advantages over traditional lighting solutions such as incandescent and fluorescent lighting because they use less energy, are more durable, operate longer, can be combined in red-blue-green arrays that can be controlled to deliver virtually any color light, and contain no lead or mercury. In many applications, one or more LED dies (or chips) are mounted within an LED package or on an LED module, which may make up part of a lighting unit, lamp, “light bulb” or more simply a “bulb,” which includes one or more power supplies to power the LEDs. An LED bulb may be made with a form factor that allows it to replace a standard threaded incandescent bulb, or any of various types of fluorescent lamps. LEDs can also be used in place of florescent lights as backlights for displays.
Color reproduction can be an important characteristic of any type of artificial lighting, including LED lighting. For lamps, color reproduction is typically measured using the color rendering index (CRI). The CRI is a relative measurement of how the color rendition of an illumination system compares to that of a particular known source of light. In more practical terms, the CRI is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI equals 100 if the color coordinates of a set of test surfaces being illuminated by the lamp are the same as the coordinates of the same test surfaces being irradiated by the known source. CRI is a standard for a given type light or light from a specified type of source with a given color temperature. A higher CRI is desirable for any type of replacement lamp.
To achieve accurate color, wavelength conversion material is sometimes used in lighting systems. The wavelength conversion materials may produce white light when struck by light of a specified color, or may produce an additional color of light that mixes with other colors of light to produce white light, or another specific desired color of light. As an example, phosphor particles can be used as a wavelength conversion material. Phosphor absorbs light at one wavelength and re-emits light at a different wavelength. Typically, phosphor particles are randomly distributed within the matrix of encapsulant material. The term phosphor can refer to materials that are sometimes also referred to as fluorescent and/or phosphorescent. Most phosphors absorb light having low wavelengths and re-emit light having longer wavelengths.